1. Field of the Invention
The present invention relates to a radar apparatus installed on a vehicle, and more particularly to such a radar apparatus, which is used for a tracking operation or an alarm system for prevention of collision of vehicles.
2. Description of the Related Art
For the application in a tracking operation relative to a preceding vehicle and an alarm system for prevention of collision between vehicles, a radar apparatus installed on a vehicle has been developed. In the radar apparatus, a beam such as millimeter wave and laser wave is radiated to a reflector in such a way that a reflection wave is received back from the reflector to detect information on the reflector. For use in this radar apparatus installed on a vehicle, various radars have been developed such as an FM radar which transmits and receives a frequency modulation wave, an AM radar which transmits and receives an amplitude modulation wave, and a pulse radar.
In the radar apparatus installed on a vehicle, namely, in a vehicle radar apparatus for monitoring the front side of the vehicle, a beam having a sharp directivity is mechanically scanned to cover a predetermined angle range in the front of the vehicle. Otherwise, plural beams of the sharp directivity are transmitted in order from a plurality of antennas arranged to direct the beams at slightly different directions from each other. Thus, by performing electronic scanning of the beams, an azimuth data for the reflector and a distance to the reflector may be obtained. In Japanese Patent No. 2567332, which is granted to the assignee of the present invention, a time divisional radar system is disclosed, in which a beam is electronically scanned. Then, a weighting and averaging operation is performed in accordance with the reception level of the reflection wave. Thus, it is possible to detect the azimuth angle to the reflector.
In a general radar apparatus installed on vehicle, there is a case where the signal-to-noise (SN) ratio of the reflection wave is decreased so that the reception level of the reflection wave does not reach a predetermined threshold value. Also, when a plurality of reflectors such as road side structures or peripheral vehicles exist, there is a case where a frequency shift is brought about due to mutual influences. As a result, the detection of the reflection wave becomes difficult, so that various problems are caused such as degradation of the position precision of the reflector and no detection of the reflector.